1. Field of the Invention
This invention relates to an image recording apparatus which operates by raster scanning, or a method of two-dimensional scanning exposure of a light-sensitive material that is transported in an auxiliary scanning direction as it is scanned with light beams that are emitted from exposing light sources and which are being deflected in the main scanning direction. More particularly, the invention relates to an image recording apparatus that employs exposing optics equipped with a dust excluding mechanism which prevents the optical paths of the light beams from being blocked by dust particles.
2. Description of the Background Art
In a raster scan based image recording apparatus, light beams emitted from exposing light sources of three colors, red (R), green (G) and blue (B), are first shaped to parallel light by means of collimator lenses associated with the respective exposing light sources R, G and B. The collimated light beams are then allowed to be incident on a light deflector such as a polygonal mirror to be deflected in the main scanning direction and, thereafter, they are adjusted by an f.theta. lens such that they are incident on the light-sensitive material to form a focused image of a specified shape in a specified position.
The light-sensitive material is withdrawn from a magazine in which it is contained in a suitable form such as a roll and subsequently transported by a suitable means such as a auxiliary scanning transport means at a specified speed in the auxiliary scanning direction which is perpendicular to the main scanning direction. The light-sensitive material in a web form that has been transported in the auxiliary scanning direction to the image recording position by the auxiliary scanning transport means is subjected to two-dimensional scanning exposure with the light beams from the exposing light sources R, G and B that are being deflected in the main scanning direction, whereby a latent image is recorded on the entire surface of the light-sensitive material.
In most cases, the exposing optics in the image recording apparatus of the raster scan type employs exposing light sources that are capable of emitting light beams at wavelengths in a specified narrow range, such as semiconductor lasers (LD), light-emitting diodes (LED) and other light-emitting devices that have advantages such as low cost and small size.
Mirrors, lenses and other optical elements used in the exposing optics in the image recording apparatus are usually sealed within an enclosure to prevent dust adhesion on their surfaces. The enclosure is provided with an entrance window in the area through which the light beams from the respective exposing light sources are admitted into the enclosure and also provided with an exit window in the area through which the light beams will eventually emerge from the enclosure.
In practice, however, the edges of the light-sensitive material are abraded during transport in the image recording apparatus to generate dust particles and, in addition, the various kinds of dust in the air which include the dust particles originating from the light-sensitive material are adsorbed on the surface of the beam exit window on the enclosure for various reasons, including the contact and friction of the light-sensitive material with the rollers between which it is held for transport and the static charges that build up in the exposing optics due, for example, to the rotation of the polygonal mirror. The dust adsorbed on the surface of the exit window partly blocks the optical paths of the light beams issuing from the respective exposing light sources and their amounts are reduced to cause a serious problem, i.e., unevenness in density develops as streaks in those areas of the recorded image which have received the smaller quantity of light.
One of the means that have been proposed with a view to solving this problem is an image recording apparatus that employs exposing optics having a dust excluding (dustproof) mechanism which ensures that the optical paths of the light beams emitted from the exit window on the enclosure of the optical elements will not be blocked by dust particles.
The concept of an exemplary dust excluding mechanism for use in this type of exposing optics is shown in FIG. 5. The dust excluding mechanism generally indicated by 80 comprises a duct 62 having an entrance for an air stream, an entrance for light beams and an exit common to the air stream and light beams, a fan 64, a filter 66 and a shutter 68 for closing the exit of the duct 62 which is common to the air stream and light beams.
In the illustrated dust excluding mechanism 80, the light beam entrance of the duct 62 is fixed to the exit window 44 on the enclosure 40 of the optical elements through which the light beams from the exposing light sources will emerge, and the shutter 68 is fitted at the exit of the duct 62 which is common to the air stream and light beams. The fan 64 is fitted at the entrance of the duct 62 through which the air stream is admitted, whereas the filter 66 is detachably fitted downstream of the fan 64 in the direction of the air stream.
The shutter 68 fitted at the exit of the duct 62 which is common to the air stream and light beams is opened only in the case of actual scanning exposure. Thus, the shutter 68 is closed both in a standby mode where scanning exposure is not performed and in a power OFF mode so that dust particles will not get into the duct 62.
To perform scanning exposure, the shutter 68 is opened and, at the same time, the fan 64 operates to supply the duct 62 with an air stream that has been freed of dust particles by means of the filter 66. Stated more specifically, a clean air stream is supplied into the duct 62 by means of the combination of the fan 64 and the filter 66 and emerges from the duct 62, with the shutter 68 opened, through the exit common to the air stream and light beams, whereby the entrance of dust particles into the duct 62 is prevented.
By providing the exposing optics with the dust excluding mechanism for preventing the entrance of dust into the duct 62, one can ensure that the dust particles will not be adsorbed on the surface of the exit window 44 on the enclosure 40 of the optical elements and that the light beams incident on the light-sensitive material after emerging from the duct 62 through the exit will not have any part of their optical paths blocked by the dust particles. As a result, one can record defect-free images of high quality that do not have any density unevenness occurring in the form of streaks due to the dust particles.
A problem with the dust excluding mechanism 80 is that depending on the environment of use, the performance characteristics of the filter 66 will deteriorate in several months and it is no longer capable of complete removal of the dust particles which are the cause of density unevenness in the form of streaks. In addition, the airflow resistance increases to lower the dust excluding effect of the filter. Therefore, filter replacements have to be made once every few months.
However, during filter replacement, dust may occasionally enter the duct 62 to be eventually adsorbed on the surface of the exit window 44, potentially causing image defects such as the density unevenness in the form of streaks.